KR101458676B1 - Positive active material for lithium secondary battery, method of preparing the same, and lithium secondary battery using the same - Google Patents

Abstract

The present invention provides a cathode active material for a lithium secondary battery comprising a base material comprising a lithium metal oxide having a layered structure and a cathode active material for a lithium secondary battery, the cathode active material comprising a surface layer material composed of a lithium metal oxide having a spinel structure, High-capacity characteristics and high capacity retention rate can be realized at the same time.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a positive active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a cathode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the same. More particularly, the present invention relates to a cathode active material for a lithium secondary battery capable of realizing high-capacity and high-rate characteristics and capacity retention ratio at the same time as a high capacity, a method for producing the same, and a lithium secondary battery using the same.

Lithium secondary batteries have been increasingly used in small electronic devices for electric vehicles and electric power storage, and there is a growing demand for cathode materials for secondary batteries having high safety, long life, high energy density and high output characteristics. LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-xy Co _x Mn _y O ₂ (0 <x + y <1), and LiMnO ₂ (Li _x Ni _y O ₂ ) are used as the cathode active material of the lithium secondary battery. ₂ Have been studied.

Lithium metal oxide, which has been attracting attention recently, is a cathode active material having a high capacity of 240 mAh / g or more per unit weight and is attracting attention as a next generation electric vehicle and a cathode material for power storage. However, when the irreversible capacity due to the phase change is large in the first charge / discharge, it is difficult to realize a specific capacity at the time of discharging, and when the content of manganese (Mn) is high, the electric conductivity decreases and the discharge capacity decreases at a high rate.

Accordingly, an object of the present invention is to provide a cathode active material for a lithium secondary battery capable of realizing high-capacity and high-rate characteristics and capacity retention ratio at the same time as a high capacity, a process for producing the same, and a lithium secondary battery using the same.

The present invention relates to a base material containing a lithium metal oxide having a layered structure represented by the following formula (1); And a surface layer material containing a lithium metal oxide having a spinel structure represented by the following Chemical Formula 2 coated on the surface of the base material.

Li _{1 + x} M _{1 - x} O ₂ (One)

(Wherein 0.05 <x <0.33, M: Co, Ni, Mn, Zr, Cr, V, Ti, Fe and Cu)

_{LiM 'y Mn 2 - y O} 4 (2)

Wherein 0 < y &lt; 0.5 and M 'is at least one transition metal selected from the group consisting of Co, Ni, Mn, Zr, Cr, V, Ti, Fe and Cu.

The present invention also provides a method for producing the above-mentioned cathode active material.

The present invention also provides a lithium secondary battery using the above-mentioned cathode active material.

The present invention provides a positive electrode active material for a lithium secondary battery in which a surface layer material containing a lithium metal oxide having a spinel structure is coated on a surface of a base material containing a lithium metal oxide having a layered structure, It is possible to provide a positive electrode active material for a lithium secondary battery having a high capacity retention rate even after repeated charging and discharging.

1 is a graph showing a 1 ^st charge discharge curve of the cathode active material prepared in Examples and Comparative Examples.
2 is an XRD structural analysis graph of the cathode active material prepared in Examples and Comparative Examples.
FIG. 3 is a graph comparing lifetime characteristics of the cathode active materials prepared in Examples and Comparative Examples. FIG.
4 is a schematic view showing an example of a lithium secondary battery according to the present invention.

The present invention relates to a base material containing a lithium metal oxide having a layered structure represented by the following formula (1); And a surface layer material containing a lithium metal oxide having a spinel structure represented by the following Chemical Formula 2 coated on the surface of the base material.

Li _{1 + x} M _{1 - x} O ₂ (One)

Wherein M is at least two transition metals selected from the group consisting of Co, Ni, Mn, Zr, Cr, V, Ti, Fe and Cu,

_{LiM 'y Mn 2 - y O} 4 (2)

Wherein 0 < y &lt; 0.5 and M 'is at least one transition metal selected from the group consisting of Co, Ni, Mn, Zr, Cr, V, Ti, Fe and Cu.

That is, the cathode active material according to the present invention has a structure in which a surface layer material having a spinel structure is formed on the surface of a base material containing a lithium metal oxide having a layered lithium excess amount of lithium having a high capacity, It is possible to provide a positive electrode active material having a high capacity retention rate even after it is repeated.

The surface layer material may be contained in an amount of 0.5 to 10% by weight based on the total weight of the cathode active material. When the amount of the surface layer material is less than 0.5% by weight based on the total weight of the cathode active material, the effect of improving the high-rate characteristics and the lifetime characteristics due to the formation of the surface spinel structure is insignificant. When the amount exceeds 10% by weight, it's difficult.

The cathode active material may have a specific surface area of 2 to 5 m 2 / g. When the specific surface area of the cathode active material is less than 2 m &lt; 2 &gt; / g, the moving area of Li ions is narrowed and the discharge capacity is remarkably lowered at a constant current density. When the specific surface area exceeds 5 m & It may be difficult to realize a uniform mixture.

The lithium metal oxide of the layered structure preferably contains Co, Ni and Mn, and is preferably Li _{1 .17} Ni _{0 .17} Co _{0 .17} Mn _{0 .49} O ₂ , Li _{1 .17} Ni _{0 .33} Co _{0 . 05} Mn _{0 .46} O ₂ or _{Li 1 .17 Ni 0 .34 Co 0} .03 Mn of _{0 .47} O ₂ is especially preferred. In addition, the lithium metal oxides of the spinel structure and may include a Mn, LiNi _{0 .5} particularly preferably _{Mn 1 .5 O 4, LiMn 2} O 4.

The present invention also provides a method for manufacturing a lithium secondary battery, comprising: mixing a lithium compound and a first transition metal compound, followed by a first calcination to form a base material containing a layered lithium metal oxide; Forming a second transition metal layer on a surface of the base material; And a step of mixing the base material on which the second transition metal layer is formed with a lithium compound and then performing secondary firing to form a surface layer material containing a lithium metal oxide having a spinel structure. The method for producing a cathode active material for a lithium secondary battery of the present invention .

The lithium compound may be Li ₂ CO ₃ or LiOH and the first transition metal compound may be at least one transition metal selected from the group consisting of cobalt, nickel, manganese, zirconium, chromium, vanadium, titanium, iron, A compound containing a metal may be used.

In the method for producing a cathode active material, the first firing is preferably performed at a temperature of 700 to 900 DEG C for 15 to 35 hours. If the calcination temperature is less than 700 ° C, the crystallinity is low and the life characteristics may be deteriorated. If the calcination temperature is higher than 900 ° C, the particle size may be large and the specific surface area may be small, resulting in deterioration of capacity and rate characteristics.

The lithium compound and the first transition metal compound can be mixed in a chemical equivalent ratio ranging from 1.20: 1 to 1.50: 1. Within the above range, it is possible to improve the capacity of the battery by preparing an excess lithium metal oxide.

In the second transition metal layer formation step, the base material is added to and mixed with the second transition metal compound aqueous solution or the organic solvent, and then sodium carbonate (Na ₂ CO ₂ ) solution, sodium hydroxide (NaOH), ammonium persulfate (NH ₄ ) ₂ S ₂ O ₈ ] is added and reacted at a temperature of 40 ° C to 80 ° C for 2 hours to 10 hours.

The second transition metal compound may be a compound containing at least one transition metal selected from the group consisting of cobalt, nickel, manganese, zirconium, chromium, vanadium, titanium, iron and copper.

The surface layer material forming step may be performed by mixing the base material having the second transition metal layer formed thereon with a lithium compound and then performing secondary firing at a temperature of 300 to 700 ° C for 5 to 35 hours to form a lithium metal oxide having a spinel structure . If the secondary firing temperature is higher than 700 캜 and higher than the first firing temperature, the characteristics of the active material may be changed, which may cause deterioration of properties. If the secondary firing temperature is less than 300 캜, Li and the transition metal compound do not react to form a spinel structure. The lithium compound may be the same as that used in the base material forming step.

In addition, the present invention provides a lithium secondary battery including the above cathode active material.

The lithium secondary battery may further include a positive electrode including a positive electrode active material, a negative electrode, a separator, and a non-aqueous electrolyte. The structure and the manufacturing method of the secondary battery are known in the technical field of the present invention and can be appropriately selected without departing from the scope of the present invention.

For example, the positive electrode is prepared by applying a composition for forming a positive electrode active material containing a positive electrode active material and a binder according to the present invention to a positive electrode collector, drying and then rolling.

The binder serves to bind the positive electrode active materials and to fix them to the current collector. Any binders used in the technical field can be used without limitation, and preferable examples include polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl And may be at least one selected from the group consisting of polyvinylpyrrolidone, polyvinylpyrrolidone, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, polyethylene, polypropylene, styrene butylene rubber and fluorine rubber.

The composition for forming a positive electrode active material may further include a solvent such as NMP (N-methyl-2-pyrrolidone) and an oligomer such as polyethylene or polypropylene to the positive electrode active material and the binder; A filler made of fibrous materials such as glass fibers, carbon fibers, and the like. Further, it may further include a conductive material shown in the following cathode.

The positive electrode collector may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon; Surfaces of copper and stainless steel surface treated with carbon, nickel, titanium or silver; Aluminum-cadmium alloy, and the like, and various shapes such as a film, a sheet, a foil, a net, a porous body, a foaming agent, and a nonwoven fabric can be used.

The negative electrode may be manufactured by applying and drying a composition for forming an anode active material on a negative electrode current collector, or may be lithium metal. The composition for forming the negative electrode active material may further include a binder and a conductive material.

The negative electrode active material may be selected from the group consisting of carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon, lithium and silicon, aluminum (Al), tin (Sn), lead (Pb) Metal alloys capable of alloying such as bismuth (Bi), indium (In), manganese (Mg), gallium (Ga), cadmium (Cd), silicon alloys, tin alloys, aluminum alloys and the like, And the like.

The negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon; Surfaces of copper and stainless steel surface treated with carbon, nickel, titanium or silver; Aluminum-cadmium alloy, and the like, and various shapes such as a film, a sheet, a foil, a net, a porous body, a foaming agent, and a nonwoven fabric can be used.

The separation membrane is disposed between the cathode and the anode, and includes an olefin-based polymer such as polypropylene; For example, polyethylene, polypropylene, polyvinylidene fluoride or a multilayer film of two or more thereof, a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene A three-layer separator, a mixed multilayer film such as a polypropylene / polyethylene / polypropylene three-layer separator, or the like.

The nonaqueous electrolyte solution may include a nonaqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, and the nonaqueous organic solvent may include, for example, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate Methyl ethyl ketone, diethyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate,? -Butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, But are not limited to, 1,2-dioxane, 2-methyltetrahydrofuran, acetonitrile, dimethylformamide, N-methyl-2-pyrrolidinone, dimethylsulfoxide, , Methyl sulfolane, and the like.

The organic solid electrolyte may be a gelated polymer electrolyte impregnated with a polymer electrolyte such as polyethylene oxide, polyacrylonitrile or the like, and the like.

Wherein the inorganic solid electrolyte is selected from the group consisting of Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI -LiOH, Li ₃ PO ₄ -Li ₂ S-SiS _{2, and} other nitrides, halides, sulfates and the like of Li.

The lithium secondary battery can be separated into a coin type, a prismatic type, a cylindrical type, a pouch type, and the like. Since the structure and manufacturing method of these batteries are known in the art, detailed description thereof will be omitted. The coin-type lithium secondary battery, also referred to as a coin cell, is exemplarily shown in Fig. In the case of such a coin type lithium secondary battery, as shown in Fig. 4, an electrode (current collector 12, electrode material 13), a separator film 14, an electrode (electrode material 13 and the current collector 12 are successively laminated and impregnated with an electrolytic solution and sealed with a metal lid 15 and a gasket 16 can be mentioned. The basic configuration of the square type, the cylindrical type, the pouch type, and the like is the same as that of the coin type battery, and it may vary depending on purposes or characteristics (shape, specification, etc.) The detailed description of the remaining structures and manufacturing methods thereof will be omitted.

Hereinafter, the present invention will be described in more detail with reference to the following examples and comparative examples. The following examples are only illustrative of the present invention and do not limit the scope of protection of the present invention.

[ Synthetic example  1] Synthesis of first transition metal compound

Nickel sulfate (NiSO ₄ ), cobalt sulfate (CoSO ₄ ) and manganese sulfate (MnSO ₄ ) were dissolved in water at a ratio of 2: 2: 6 and then added to a 1 M NaOH solution. Ammonia water (NH ₄ OH) of the same equivalent ratio as the metal concentration ratio was slowly added to the above solution. After 12 hours by using a continuous-type reactor, the precipitate formed was filtered off, was washed 10 times with distilled water, dried in a drying oven at a temperature of _{120 ℃ Ni 0 .2 Co 0 .2} Mn 0 .6 ( OH) ₂ was obtained.

[ Synthetic example  2] Synthesis of layered lithium metal oxide

The transition metal compound (nickel cobalt manganese hydroxide; Ni _0.2 Co _0.2 Mn _0.6 (OH) ₂ ) and lithium carbonate (Li ₂ CO ₃ ) of Synthesis Example 1 were mixed so that the chemical equivalence ratio of lithium and the transition metal was 1.40: 1 in 800 ℃ baked for 24 hours to obtain a lithium metal oxide _{(Li 1 .17 Ni 0 .17 Co} 0 .17 Mn 0 .49 O 2) of the layered structure is used as the base material. The specific surface area of the obtained positive electrode active material was 3 m 2 / g.

[ Example  1] 1.0 wt% Synthesis of cathode active material of surface material / base material structure

MnSO ₄ The lithium metal oxide having the layered structure synthesized in Synthesis Example 2 above was slowly added to the aqueous solution for 60 minutes using a magnetic bar. A Na ₂ CO ₃ aqueous solution was slowly added to the above mixed solution and reacted at 60 ° C for 5 hours to form a layer of MnCO ₃ as a transition metal layer on the surface of the base material made of the layered lithium metal oxide. The resulting transition metal layer-based material slurry was filtered with a filter, washed and then dried in an oven at 120 ° C. for 12 hours to obtain a powder of the structure of the transition metal layer-based material. To the thus obtained powder was added Li ₂ CO ₃ was mixed so that the chemical equivalent ratio of lithium (Li) to manganese (Mn) was 1: 2, and then calcined at 700 ° C for 20 hours to obtain LiMn ₂ O as the cathode active material of the surface material / _{4 was contained in} an amount of 1.0% by weight.

[ Example  2] 2.0 wt% Synthesis of cathode active material of surface material / base material structure

Except that the content of LiMn ₂ O ₄ was adjusted to 2.0 wt%, thereby obtaining a cathode active material having a surface material / base material structure according to the present invention.

[ Example  3] 10.0 wt% Synthesis of cathode active material of surface material / base material structure

Except that the content of LiMn ₂ O ₄ was adjusted to 10.0% by weight in the same manner as in Example 1, to obtain a cathode active material having a surface layer material / base material structure according to the present invention.

[ Example  4] 0.2% by weight LiMn ₂ O ₄  Synthesis of mixed cathode active material

A cathode active material having a surface material / base material structure was obtained in the same manner as in Example 1, except that the content of LiMn ₂ O ₄ was 0.2 wt%.

[ Example  5] 12% by weight of LiMn ₂ O ₄  Synthesis of mixed cathode active material

A cathode active material having a surface material / base material structure was obtained in the same manner as in Example 1 except that the content of LiMn ₂ O ₄ was 12 wt%.

[ Comparative Example  One]

But as a base material using a lithium metal oxide substrate material _{(Li 1 .17 Ni 0 .17 Co} 0 .17 Mn 0 .49 O 2) of the laminated structure prepared in Synthesis Example 2, the lithium metal of the spinel structure, the surface layer material A cathode active material containing no oxide was used.

[ Comparative Example  2] 2% by weight of LiMn ₂ O ₄  Synthesis of mixed cathode active material

98.0 wt% of the layered lithium metal oxide synthesized in Synthesis Example 2 and 2.0 wt% of a lithium metal oxide (LiMn ₂ O ₄ ) having a spinel structure were mixed in a powder mixer (NOB-mini, product of Hosokawa Industrial Co., Ltd.) The mixture was mixed with 2.0 wt% of LiMn ₂ O ₄ Mixed layered structure cathode active material was obtained.

[ Comparative Example  3] 10% by weight of LiMn ₂ O ₄  Synthesis of mixed cathode active material

90% by weight of the layered lithium metal oxide synthesized as in Synthesis Example 2 and 10.0% by weight of the spinel structure lithium metal oxide (LiMn ₂ O ₄ ) were mixed by physical methods to obtain 10.0% by weight of LiMn ₂ O ₄ Mixed layered structure cathode active material was obtained.

[ Experimental Example  One] XRD  Structure analysis

XRD structure analysis was performed on the cathode active material obtained in Example 2 and Comparative Example 1, and the results are shown in FIG. The analytical conditions were 2θ = 10 to 80 ° and 0.001 ° / step. The Cu-Kα line (1.5418 Å, 40 kV / 30 mA, manufacturer: BRUKER AXS) was measured and JCPDS (01-070-865) Respectively. As shown in FIG. 2, it was confirmed that the cathode active material of Example 2 had a spinel peak formed after coating LiMn ₂ O ₄ .

[ Experimental Example  2] Evaluation of battery characteristics

Denka Black and polyvinylpyrrolidone (PVDF) binders were mixed in the ratio of 94: 3: 3 and coated on the aluminum foil to prepare an electrode plate. 2032 coin cells were fabricated using metal lithium as the cathode and 1.3M LiPF ₆ EC / DMC / EC = 3: 4: 3 solution as the electrolyte. Charging and discharging of one cycle proceeded to 0.1 C in the range of 3.0 to 4.7 V, followed by 0.1 C discharge capacity in the range of 3 to 4.6 V, high rate characteristics of 3 C or more, And the results are shown in Table 1 and the following Table.

division Surface material
content
(weight%) 0.1 C
Charging capacity
(mAh / g) 0.1 C
Discharge capacity
(mAh / g) efficiency
(%, Charge / discharge) High rate characteristic
(%) Capacity retention rate
(%) Comparative Example 1 - 306 275 90 77 85 Example 1 1.0 303 282 93 79 91 Example 2 2.0 302 287 95 81 92 Example 3 10.0 290 281 97 83 92 Example 4 0.2 306 272 89 77 84 Example 5 12.0 286 266 93 78 86 division Mixed LiMn ₂ O ₄
content
(weight%) Primary
Charging capacity
(mAh / g) Primary
Discharge capacity
(mAh / g) efficiency
(%, Charge / discharge) High rate characteristic
(%) Capacity retention rate
(%) Comparative Example 2 2.0 300 276 92 78 88 Comparative Example 3 10.0 284 261 92 79 87 High rate characteristic (%) = 3C / 0.3C
Capacity retention rate = capacity retention rate after 50 cycles (1C charge discharge)

As shown in Table 1, in the case of the cathode active material according to the present invention, as the content of the lithium metal oxide (LiMn ₂ O ₄ ) of the spinel structure increases, the charging capacity decreases but the efficiency becomes better. . As the content of lithium metal oxide (LiMn ₂ O ₄ ) in the spinel structure increases, the high-rate characteristics exhibited by the 3C capacity relative to the 0.3C capacity were improved. the positive electrode active material _{(Li 1 .17 Ni 0 .17 Co} 0 .17 Mn 0 .49 O 2) was confirmed that contrast enhancement.

On the other hand, in the case of the cathode active material in which the spinel structure and the layered lithium metal oxide were combined using the dry mixing method as in Comparative Examples 2 and 3, the charging / discharging efficiency, the rate characteristic and the lifetime characteristics were improved, 2 and 3, it was confirmed that the improvement of the characteristics was insignificant.

Referring to FIG. 2, it can be seen that a flat region occurs in the 2.8V region due to the coating of LiMn ₂ O ₄ . It is also confirmed that the capacity is increased in Examples 2 and 3 as compared with Comparative Examples 2 and 3.

That is, it was confirmed that the lithium secondary battery using the cathode active materials of the examples according to the present invention exhibited higher rate characteristics and capacity retention rates after 50 cycles than the comparative examples.

Claims (11)

A base material containing a layered lithium metal oxide represented by Li _1.17 Ni _0.17 Co _0.17 Mn _0.49 O ₂ , Li _1.17 Ni _0.33 Co _0.05 Mn _0.46 O ₂ or Li _1.17 Ni _0.34 Co _0.03 Mn _0.47 O ₂ ; And a surface layer material containing a lithium metal oxide having a spinel structure represented by the following formula (2) coated on the surface of the base material:
_{LiM 'y Mn 2-y O} 4 (2)
Wherein 0 < y &lt; 0.5 and M 'is at least one transition metal selected from the group consisting of Co, Ni, Mn, Zr, Cr, V, Ti, Fe and Cu. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the surface layer material is contained in an amount of 0.5 to 10% by weight based on the total weight of the positive electrode active material. The positive electrode active material for a lithium secondary battery according to claim 1, wherein the positive electrode active material has a specific surface area of 2 to 5 m 2 / g. delete delete The positive electrode active material for a lithium secondary battery according to claim 1, wherein the lithium metal oxide represented by Formula 2 is LiMn ₂ O ₄ or LiNi _0.5 Mn _1.5 O ₄ . Mixing a lithium compound and a first transition metal compound and then performing a first calcination to form a base material containing a layered lithium metal oxide;
Forming a second transition metal layer on a surface of the base material; And
Mixing a base material on which the second transition metal layer is formed with a lithium compound and then performing secondary firing to form a surface layer material containing a lithium metal oxide having a spinel structure;
The method of producing a cathode active material for a lithium secondary battery according to any one of claims 1 to 3, 8. The method of claim 7, wherein the mixing of the lithium compound and the first transition metal compound is performed such that the chemical equivalent ratio of lithium to transition metal is 1.20: 1 to 1.50: 1. 8. The method of claim 7, wherein the primary firing is performed at a temperature of 700 to 900 DEG C for 15 to 35 hours. 8. The method of claim 7, wherein the secondary firing is performed at a temperature of 300 to 700 DEG C for 5 to 35 hours. A lithium secondary battery comprising the cathode active material according to any one of claims 1 to 3 and 6.

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